Conventional drag bits used in, for example, continuous mining machines, saw blades, auger drills, longwall shearers and the like are basically of a shape wherein only the tip presents an effective cutting edge while the remainder of the cutting face effectively pushes the material out of the path of the bit. Examples of cutting tools of this general type include those disclosed in U.S. Pat. Nos. 2,690,904 (Muschamp et al) and 4,804,231 (Buljan et al). The former patent discloses a cutter pack assembly for reversible chain mining machines which employs conventional straight edge pick points while the latter discloses a point attack style mine and milling tool.
Further patents of interest are because of the shaped of the cutters or cutter members provided include U.S. Pat. Nos. 4,593,777 (Barr), 4,559,753 (Barr), and 4,538,690 (Short, Jr.). Before considering these patents in more detail, the nomenclature used in describing cutters and bits requires some clarification. The Barr and Short, Jr. patents describe a drag bit and associated cutters, with the overall device being referred to as the drag bit. This drag bit is equipped with a series of cutter elements that actually cut the rock and these are referred to as cutting members or simply cutters. This is a common arrangement for rotary drill bits. However, in the field of excavation the nomenclature is somewhat different. More particularly, the overall device which mounts the cutting elements is generally referred to as the cutterhead while the individual cutting elements are referred to as bits. Thus, the individual drag bits of the present application correspond to the cutting members of cutters of the Short, Jr. and Barr patents. Further, the devices disclosed in those patents concern a particular type of cutterhead which is referred to as a drag type drill bit and which is used with a rotary drilling machine to drill relatively small diameter holes, typically up to 24 inches in diameter.
With this background, it is noted that the cutters of the Short, Jr. and Barr patents are broadly relevant to the present invention to the extent that these cutters are concave in shape but are otherwise irrelevant. The cutters of these patents are combined with a thin, hard coating (e.g. PDC) which is said to yield superior wear characteristics over conventional-flat face cutters. In particular, these patents state that the PDC coating maintains a sharp cutting edge even as the edge wears and that the rake angle of the concave shape changes as the edge wears, so that the changing rake angle allegedly provides an optimum match for different types of rocks encountered.